This invention relates to a method for producing metal lithographic printing plates. More particularly, this invention relates to a novel method for producing an improved metal substrate useful in the production of metal, presensitized lithographic printing plates. Even more particularly, this invention relates to a novel method for the production of an improved aluminum sheet substrate useful in the production of aluminum, presensitized lithographic printing plates.
Heretofore, in the production of metal presensitized lithographic printing plates, it had been found beneficial to treat the surface of the metal substrate sheet, with a protective interlayer substance which imparts beneficial characteristics to the final lithographic printing plate thus produced. The prior art teaches that it is desirable to treat the metal sheet substrate surface receiving the light sensitive coating material, which when exposed to light and developed becomes the printing surface of the printing plate, with an undercoating substance that forms a strong bond with the metal sheet substrate and with the light sensitive coating material.
Many such undercoating treatments are known in the art for manufacturing longer-running lithographic plates, and can be used on the sheets of this invention. U.S. Pat. Nos. 3,160,506, 3,136,636, 2,946,683, 2,922,715 and 2,714,066 disclose a variety of suitable materials for undercoating bonding substances onto plates and methods for applying them. Alkali silicate, silicic acid, alkali zirconium fluoride and hydrofluozirconic acid solutions presently are the most important commercial bonding substances. Those materials substantially improve the bonding of the light-sensitive coating to the underlying metallic base which otherwise generally tends to have inadequate affinity for the coating. Of the various known bonding materials, the Group IV-B metal fluorides, the alkali metal salts and the acids thereof are preferred. In particular, the alkali zirconium fluorides, such as potassium zirconium hexafluoride, and hydrofluozirconic acid disclosed in U.S. Pat. Nos. 3,160,506 and 2,946,683 are especially satisfactory for preparing anodized aluminum bases to receive a light-sensitive coating.
The undercoating substance is usually applied to the metal sheet substrate by immersing the sheet in a solution of the bonding substance. This requires that an immersion tank be employed, and depending upon the rate of speed of the moving web of the metal substrate, the length of detention time within the immersion tank can be varied. However, it is recognized that an immersion tank of substantial size is required to obtain a satisfactory treatment with the bonding substance in prior art practice. By the instant invention it has been found that the need for an immersion tank can be eliminated, thus producing a concomitant reduction in the amount of energy required to obtain equivalent or superior results than heretofore possible in prior art processes.
In general, the instant invention comprises a method whereby the undercoating substance is applied solely to and directly on the surface of the metal sheet substrate which is employed for use as a lithographic printing plate. More particularly, a solution of the undercoating substance is applied to the surface of the metal sheet substrate in sufficient concentration to provide for complete reaction with the surface to give a satisfactory bonding coating to the metal sheet substrate. The thus applied solution is then subjected to a heating treatment at elevated temperature whereby the surface of the metal sheet substrate is brought quickly to dryness. This treatment concurrently increases the concentration of the bonding substance on the surface of the metal sheet substrate to a satisfactory level and rapidly brings the reaction between the solution and substrate material to completion and forms a novel coated substrate with markedly improved properties. The surface may then be treated to remove excess unreacted materials, and the resultant metal sheet substrate may then be further treated in various manners known to the art to produce presensitized lithographic printing plates.